Friction!
Physics 40s
Friction
Ff
The magnitude of the force of surface friction is
the product of the coefficient of friction and the
magnitude of the normal force.
The direction of the force of friction is always
opposite to the direction of the motion.
Friction
• Definition: the force that opposes the
motion of one surface past another
• Dependent upon: 1) type of material
• 2) how tightly pressed they are together
(more weight =more
• friction)
• It’s time to experience friction, so rub
your hands together.
Why would a snowboarder wax the board?
What conditions would change the frictional force as they
move downhill?
Characteristics of
friction

ALWAYS opposes motion

a) it will slow it

b) or prevent it from moving


Creates heat
3) Symbol is lowercase ‘f’…Ff
2 Types of Friction
Static
1)
2)

prevents motion from starting.
Kinetic
friction
1)
slows objects down.
2)
occurs during motion
occurs before motion
Which is bigger…static or kinetic?
It is easier to keep an object
moving than start it from rest

New Symbol

μ – Greek letter “Mu”
Coefficient of friction.
– μs – coefficient of static friction
– μk – coefficient of kinetic friction
– depends on materials used and their surface
conditions
 Coefficient of friction: Decimal between 0.0 and 1.0,
unitless.


Formula for solving friction:
Ff = μ FN
Application Problems
Sand is often placed on an icy road
because the sand:
Decreases the coefficient of friction between the tires of a
car and the road
Increases the coefficient of friction between the tires of a
car and the road
Decrease the gravitational force on a car
Increases the normal force of a car on the road
An empty cart is being rolled across a
warehouse floor. If the cart was filled, the
force of kinetic friction between the cart and
the floor would
Decrease
Increase
Remain the same
A 24 kg crate initially at rest on a horizontal
floor requires a 75N horizontal force to set it in
motion. Find the coefficient of static friction
between the crate and floor.
Once the crate is in motion, a horizontal force
of 53N keeps it moving with constant velocity.
Find the coefficient of kinetic friction between
the crate and floor.
Static Friction
Reaction force to anything trying to start motion
•Equal
and opposite to applied force.
•DOES
NOT EXCEED THE APPLIED FORCE, but is equal to
it.
•Equal
and opposite to applied force, until reaches maximum
value and motion starts.
•friction “breaks”
when F is great enough and motion begins.
Static Friction
Static Frictional Force Breaks at a certain value:
fs = s FN
fs = force of static friction
s = coefficient of static friction
FN = Normal force

s is a given value. It depends on the object and the surface.
Static Friction
fs = force of static friction
s = coefficient of static friction
FN = Normal force (usually weight)
Normal force is usually just the
weight of the object.
FN = Mass* 9.80 m/s2
IMPORTANT!!!!!
If the surface is not horizontal use trig. Multiply by cos of the angle of
incline.
Almost always:
General Notes
on Friction
μs > μk
It is easier to keep an object moving
than it is to start from rest. Think
about pushing a car.
Both are almost always less than 1. If it
was greater than one, it would be
easier to pick the object up and carry
it than it would be to push it across
the flat surface (something like velcro)
Problem

Box:

How much force is needed to “budge” this box?

If we keep pushing that hard, what will the
acceleration be?